A motor may be combined with a compressor in a single housing to provide what is known as a motor-compressor device. The motor drives the compressor (via a shared rotating shaft supported on each end by a rotor-bearing system) in order to generate a flow of compressed process gas. When used to directly drive a compressor, such as a centrifugal compressor, the shaft is required to rotate at relatively high speeds. In addition to the heat generated by the electrical loss mechanisms that are characteristic of electric motor drivers, operating the motor-compressor device at high speeds increases windage frictional losses generated by the rotating components. If this heat is not properly managed or regulated, it will affect the performance of the motor and potentially damage the electrical insulation of the stator. For the case of machines supported on magnetic bearings, unregulated or unmanaged heat can also adversely affect any accompanying rotor-bearing systems, possibly leading to bearing damage and/or failure.
Prior similar integrated systems have used an external source of pressurized cooling gas in an open-loop cooling arrangement to manage the temperature of the motor and bearing systems. In these applications, the cooling gas is driven primarily by a pressure difference established between the source of cooling gas (typically the discharge of the compressor or an intermediate compressor stage) and the place to which the gas is allowed to flow to (typically the compressor inlet).
Alternatively, in systems that do not use the process gas to cool the motor, and in which the motor and the compressor do not share the same pressure-containing casing, an external fan or blower can circulate cooling air though a motor cooling loop. In such arrangements, the cooling gas is circulated through the motor and bearing systems to ventilate the housing and remove heat. Using an external pressurization system, however, can be problematic, especially if the external fan or blower fails during operation and the flow of cooling gas ceases, resulting in motor/bearing overheating and potential catastrophic failure.
Other prior systems have implemented a quasi-closed loop cooling system which uses a gas circulation mechanism that is machined directly into the rotating shaft. These types of systems, however, have a limited pressure rise capacity due to the selection of the blower design, and if the cooling requirements change, the shaft must be removed and redesigned.
In motor/compressor systems that handle “wet” process gas, such as is common in the upstream applications of the oil and gas industry, the leakage of liquids into the motor/bearing cavity through the radial seals arranged at each end of the compressor shaft has also presented a considerable amount of difficulty. While conventional radial seals may reduce process gas leakage from the compressor, under certain off-design operating conditions, liquid can nonetheless leak across the clearance defined between each radial seal and the rotating shaft, thereby trickling into adjacent bearing cavities and into the motor/bearing cooling circuit. The presence of liquid can potentially damage the bearings and introduce contaminants into the motor area and cooling circuit, which can eventually lead to the deterioration of system components. In many cases, dry gas seals are not robust enough to handle wet process gases and will require a complex gas conditioning and regulating system to avoid liquid ingress into the seal faces. Based on this, they are not good seal candidates, and they may fail when coming into contact with pressurized liquids.
Accordingly, there is a need for an improved more robust cooling system and radial seal system for a motor-compressor arrangement that will not be susceptible to the drawbacks of the prior systems described above.